The present invention concerns the field of spinal osteosynthesis, and more particularly the field of correction of the alignment of the vertebrae by means of a system comprising correction rods, hooks which can be fixed on the vertebrae, and transverse connection rods.
Such systems form a torsionally rigid correction frame.
As an example of the prior art, reference may be made to European patent EP95910695, published under number EP750477. This patent describes in particular a fixation hook for interconnecting a correction rod of a spinal osteosynthesis system and a rigid transverse rod and for clamping the correction rod against the transverse rod, said fixation hook comprising:                a body;        a passage defined by edge portions in said body for receiving the transverse rod, said passage having a first height at least equal to the thickness of the transverse rod, extending along the entire length of said body, opening freely out at one end and terminating at one end in an aperture defined in said body, and having a second height exceeding the thickness of the transverse rod;        a curved strip portion extending from said body so that a correction rod can lodge therein, said aperture being provided in a connection zone between said body and said curved strip portion;        a tapped hole defined in said body, which hole opens into said passage and is positioned in such a way that the axis of said tapped hole is in proximity to a free end portion of said curved strip portion;        a clamping screw inserted via a screwthread into said tapped hole in order to clamp a transverse rod in said passage on a correction rod lodged in said curved strip portion;        by which means the clamping screw, when it clamps said transverse rod, exerts a clamping force along a line which is offset in proximity to the free end portion of said curved strip portion in relation to the central axis of the correction rod in such a way as to cause a pivoting movement of the transverse rod about said correction rod.        
Another known European patent published under number EP446092 describes another device for rigid transverse connection between two spinal osteosynthesis rods. This device comprises two fixation elements, each consisting of a hook which is adapted to be able to engage on a rigid transverse rod in a sliding manner, and equipped with means for locking it on the transverse rod. This hook is made up of a body and two blades separated by a gap having a width corresponding to that of the transverse rod, and a support bearing for the hook on the transverse rod is formed on the body between the blades, which blades extend on each side of the transverse rod when the hook straddles the latter. Two hooks combined with a rectangular transverse rod form a relatively simple transverse connection device which can be fitted in place quickly and has a high degree of rigidity in torsion and in flexion.
The transverse connection systems of the prior art require perfect parallelism of the two connection elements formed by rods or plates. In the case where the rods are not parallel, the surgeon has to bend the transverse connection element to adapt the fixation elements on the connection elements.
The solutions in the prior art permit free fixation in the frontal plane or in the sagittal plane, or in a combination of these two planes of rotation. However, they do not permit all of the desired relative orientations. They do not make it possible to avoid the operation of adapting the transverse elements by torsion, or to avoid arrangements which are not favorable to the overall rigidity of the osteosynthesis system in relation to the prominence of the implants.